Mounting Technique Aids MMIC Performance

Performance levels of high-frequency monolithic-microwave integrated circuits (MMICs) are often compromised within packages and circuits. Fortunately, a novel technique for mounting MMICs in microstrip circuits helps optimize device performance even at millimeter wavelengths by eliminating many of the drawbacks of traditional mounting techniques. This patented new approach helps designers achieve MMIC performance promised during wafer probing but often lost in the transition to microwave/millimeter-wave circuitry.

In this new approach, a MMIC is inserted into a laser-drilled pocket in a polytetrafluoroethylene (PTFE) circuit board, establishing continuity with the groundplane and reducing or eliminating jumper wires. A secondary FR4 circuit board is bonded to the bottom of the PTFE board, from which DC control lines are connected to the chip through viaholes. In addition, the MMIC is flush with the top of the board (rather than on top of it) where it is not subject to damage from handling. Test results indicate that performance of pocket-mounted MMICs differs little from manufacturers' bare-die specifications.

The transition between a MMIC and its supporting microstrip circuit elements and RF and DC connections is the crucial factor in determining its performance when mounted in a circuit. MMIC manufacturers supply copious performance specifications for their die, which in an ideal mounting situation could be perfectly preserved. However, the typical microstrip circuit board onto which a MMIC is bonded presents a less-than-ideal environment, since the path to ground is routed to the chip from the groundplane with via holes. The resulting discontinuities cause significant mismatch, and parasitic capacitance and inductance caused by bond wires are extremely difficult to remove. Consequently, the specified performance of the device can be significantly reduced. The severity of this situation increases with frequency, and becomes a major problem at millimeter wavelengths.

A bare-die MMIC mounted on top of a circuit board is also vulnerable to damage, since it is higher than the surrounding surfaces. This issue can be addressed by incorporating the device in a ball-grid-array (BGA) or low-temperature-cofired-ceramic (LTCC) package, but in addition to being more expensive and difficult to assemble, these packages can increase insertion loss. Faced with a customer requirement for use of bare die to achieve the highest level of performance in a millimeter-wave assembly, KDI Integrated Products (Whippany, NJ) investigated various ways to mount MMICs that would retain their performance, creating a nearly pure resistance between the MMIC and its connections.

The solution involved creating a pocket for the device in the microstrip laminate just slightly larger than the device itself. This pocket not only provides electrical benefits, but also lowers a 6-mil-thick MMIC to the level of the board, reducing susceptibility to damage. To create the pocket, the top center conductor of the board is removed by laser drilling in the spot where the MMIC will be placed. The drilling continues through the dielectric layer below the center conductor, revealing the main groundplane used by the remainder of the circuit. The surface of the center conductor and groundplane are plasma-etched to remove burned material.

The microstrip and ground lines are then plated with a 0.05-mil layer of gold where the bond wires are to be connected and via holes will be placed. The metal patch on the bottom of the MMIC is attached to the groundplane with liquid silver-filled epoxy, which provides high mechanical strength and an excellent conductive path from the device to ground. Solder that is not affected by metal plating (such as indium solder), can also be used in place of epoxy, in which case the solder is placed in the pocket, and the chip is placed in the pocket over the solder. The entire assembly is heated to the solder's melting point to achieve the necessary mechanical and electrical bonding of the chip to the groundplane. The bond wires are then connected to the chip and metal-plated areas.

Pocket-mounting allows the groundplane used by the rest of the circuit to become the groundplane for the MMIC as well (Fig. 1). This direct ground connection eliminates the traditional need to provide a ground path to the bottom of the chip from the underlying groundplane through via holes. The discontinuity created by this noncontiguous path degrades circuit performance by creating inductance. This series inductance, when added to the inductance incurred from the RF input and output connections, makes tuning extremely difficult. Tuning of the device when pocket-mounted requires attention only to the wire bonds.

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When the MMIC is mounted in the pocket, inductance is minimized from the microstrip ground to the groundplane of the chip. The input port and output port bond wires can also be made extremely short because the chip surface is in the same plane as the microstrip transmission line. By reducing these "four inductances" for any input to any output of a MMIC, the best possible performance can be achieved.

The lines to the MMIC for DC bias and control are accommodated in a convenient way by bonding the bottom of the PTFE board to another board made of FR4. The lines are etched on the FR4 board and are brought up to the top surface alongside of the MMIC with vias. In most applications this eliminates jumpers on the microwave side of the circuit board, ensuring no disruption of the microwave circuit. If the MMIC requires a bypass capacitor, it can be established on the top surface with vias to the groundplane. A hermetic enclosure can be provided by welding the cover around the housing and using hermetic connectors.

The pocket-mounting technique was used in the fabrication of a 2 × 16 transmit/receive switch used to feed a Rotman or Lunenberg lens antenna array for operation at 28 GHz (Fig. 2). The assembly incorporates a PIN diode switch that connects a transmitter to any of 16 ports and rapidly switches from transmit to receive while maintaining high isolation between the transmitter and receiver. The 16 waveguide input/output ports are placed on an arc to feed or receive signals from the lens, and provide a narrow (8 deg.) 3-dB beamwidth in the transmit mode. In receive mode, the lens directs the signal to one of the 16 waveguide ports.

In the transmit mode, the signal passes through two single-pole, four-throw (SP4T) switches, arriving at the waveguide ports with a loss of about 7 dB. In receive mode, the signal passes through an SP4T switch, through an low-noise amplifier (LNA) with 14 dB gain, and through a second SP4T switch to the receive waveguide output. The receive-path loss is less than 7 dB with gain of 7 dB. All of the MMIC switches and LNAs were mounted in the pocket configuration, which was instrumental in delivering the required performance, while also minimizing the size of the housing. Only three shunt capacitor tuning blocks were required, in contrast to perhaps six that would be needed with a conventional approach.

The performance of PIN-diode MMIC switches and gallium-arsenide (GaAs) LNAs mounted in the pocket configuration (Fig. 3) was compared with specifications provided by their manufacturers for the bare die measured made on the wafer. Improved performance was noted over the frequency range of 27.35 to 28.35 GHz using the pocket-mounting technique with two MMICs used in the T/R switch.

The UMS CHA-2093, a two-stage LNA, provides a 3-dB noise figure and 14 dB of gain. When the MMIC was mounted on a raised ground plane using via holes to bring up the ground plane from below, considerable input and output tuning was required to achieve a flat gain response. With the chip in a pocket, the performance compared closely with the results obtained by UMS from wafer probing, including the 3 dB noise figure. The CHA-2093 in the pocket was placed very close to a model CP0558-1 six-throw switch from Alpha Industries/Skyworks in the matrix. Minimal and predictable tuning was still necessary between the two monolithic chips (each mounted in its own pocket), because they are not inherently perfectly matched.

The CP 0558-01 delivered insertion loss equal to Alpha's wafer probed results. The device was originally mounted on the plated-through vias and raised ground plane, which produced insertion loss of 2.8 to 3.0 dB. When it was pocket-mounted, the loss improved to 2.2 to 2.4 dB and less tuning was required over the 27.35-to-28.35-GHz range.

In summary, the electrical characteristics of the transition between an MMIC, its groundplane, and its connections determine how well the device will perform in the circuit. Traditional techniques for mounting a die or packaged MMIC on the surface of a microstrip circuit board make it difficult to reduce insertion loss to its lowest level, introduce parasitic capacitance and inductance, and place the device in the open where it is subject to damage in handling. In addition, the large number of jumpers needed to make connections to the chip might be impossible to accommodate in some space-critical applications.

The pocket-mounting technique eliminates these drawbacks, and without significant increase in cost delivers essentially the same MMIC performance in-circuit as when measured on the wafer. The technique can be used with any type of MMIC, or even transistors, and is not limited to applications in the millimeter-wave region. However, since the MMIC interconnect transition becomes even more critical at higher frequencies, the pocket-mounting technique can provide its greatest benefits in these applications.